Hitachi HIDIC MICRO-EH Applications Manual page 237

(2) Causes of congestion errors at normal scan
Congestion errors may occur at normal scan because of the following three possible reasons. In particular when using a
periodical scan program and an interrupt scan program together, care must be taken to create the program in such a way
that the total scan time does not exceed the congestion check time.
(a) When only a normal scan program is used
The scan time exceeded the congestion check time because the time required for one scan was too long.
Periodical system processing
END scan processing
Normal
Program execution
scan
(b) When both a normal scan program and a periodical scan program are used
The congestion check time was exceeded because the periodical scan program was executed and the normal scan
time became longer.
Periodical system
Periodical
processing
scan
Periodical program
Normal
END scan processing
scan
Program execution
(c) When both a normal scan program and an interrupt scan program are used
The congestion check time was exceeded because the interrupt scan program was executed due to an interrupt
input and the normal scan time became longer.
Periodical system processing
Interrupt
scan
Normal
scan
(3) Continuation of operation after a congestion error occurred
When the special internal output bit R7C0, which specifies whether the operation should continue after a congestion
error occurred, is turned on, the normal scan executes the scan until the end regardless of the congestion check time, and
after executing the END scan processing, executes the normal scan from the beginning again.
Periodical system processing
Normal END scan processing
scan Program execution
Figure 9.8 Operation when operation continuation at congestion error is set
However, note that this setting does not stop the execution of the scan when a congestion error occurred even when an
infinite loop is formed within the normal scan by the JMP instruction.
10 ms
Normal scan time (normal scan only)
Congestion check time
Figure 9.5 Congestion error at normal scan (a)
INT0
Figure 9.6 Congestion error at normal scan (b)
Interrupt analysis
processing
Program execution
END scan processing
Program execution
Figure 9.7 Congestion error at normal scan (c)
10 ms
The periodical system interrupt
is executed by interrupts at every
10 ms regardless of whether or
not there is a periodical scan or
interrupt scan.
The congestion check time can
be set to 20 ms to 2550 ms using
peripheral devices by the users.
10 ms
RTI RTI
INT0
Scan time
Congestion check time
10 ms
Scan time
Congestion check time
Scan time
Congestion check time
9-4
Chapter 9 PLC Operation